We were there just one day -- at Epcot on the Wednesday before Easter.
It was busy, but not as bad as I expected. I really worried that we'd only get to do one or two things all day. Standby lines for the major attractions were very long. At one point, Soarin' listed 260 minutes. (That's over 4 hours!) However, we were able to hit them all by utilizing FastPass. There were also a number of things that had pretty short waits -- Journey Into Imagination, the Nemo ride at the Seas, Captain EO, etc. were all walk-on or very short waits. Spaceship Earth was about 25 minutes (in the late afternoon. It had been longer earlier in the day). We also watched a couple of the performances in the World Showcase and enjoyed those as a nice break.
We normally go during less busy times and I was really worried that we would spend all day waiting in lines. We didn't get to do as much as we normally do and there was definitely more waiting than usual... but when we left everyone said they'd gotten to do "everything they'd really wanted to do" so it was successful. Spring Break probably wouldn't be my *first* choice of weeks, but I would go back again and be confident that (with flexibility and planning) we could still have a really good time.